LTRIM for list capping in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this LTRIM command?

Given a Redis list mylist with elements [a, b, c, d, e], what will be the content of mylist after running LTRIM mylist 1 3?

Redis

LTRIM mylist 1 3

A [a, d, e]

B [a, b, c]

C [c, d, e]

D [b, c, d]

💡 Hint

LTRIM keeps elements from the start index to the end index inclusive.

LTRIM trims the list to keep only elements from index 1 to 3, which are b, c, and d.

🧠 Conceptual

intermediate

Why use LTRIM for list capping in Redis?

What is the main reason to use the LTRIM command when managing Redis lists?

A To limit the list size by removing older elements and keep only the most recent ones.

B To sort the list elements alphabetically.

C To convert the list into a set.

D To duplicate the list elements for backup.

💡 Hint

Think about how to keep a list from growing too large.

LTRIM is used to keep only a specific range of elements, effectively capping the list size by removing older or unwanted elements.

📝 Syntax

advanced

Which LTRIM command syntax correctly caps a list to the last 100 elements?

You want to keep only the last 100 elements of a Redis list named events. Which command correctly achieves this?

A LTRIM events -100 -1

B LTRIM events 0 99

C LTRIM events 1 100

D LTRIM events -99 0

💡 Hint

Negative indexes count from the end of the list.

Using LTRIM events -100 -1 keeps the last 100 elements because -100 is the 100th from the end and -1 is the last element.

🔧 Debug

advanced

Why does this LTRIM command not cap the list as expected?

You run LTRIM logs 1 100 on a Redis list logs with 150 elements, but the list now has only 100 elements afterward. Why?

A LTRIM requires negative indexes to work properly.

B LTRIM does not modify the list if the end index is greater than the list length.

C LTRIM uses zero-based indexing, so starting at 1 skips the first element and trims everything after index 100, capping the list at 100 elements.

D LTRIM only works on sets, not lists.

💡 Hint

Remember how Redis indexes list elements.

LTRIM keeps elements from index 1 to 100, which is 100 elements but excludes the first element. Elements beyond index 100 are also removed, capping the list to exactly 100 elements.

❓ optimization

expert

Efficiently capping a high-traffic Redis list with LTRIM

You have a Redis list chat_messages that receives thousands of new messages per second. To keep only the latest 1000 messages, which approach is the most efficient and safe to avoid race conditions?

A Use <code>LTRIM chat_messages 0 999</code> before every <code>LPUSH</code> command.

B Run <code>LPUSH chat_messages new_message</code> followed immediately by <code>LTRIM chat_messages 0 999</code> in a single Redis transaction or pipeline.

C Use <code>LRANGE chat_messages 0 999</code> to get the latest messages and delete the rest manually.

D Run <code>LPUSH chat_messages new_message</code> and then run <code>LTRIM chat_messages 0 999</code> as separate commands with delay.

💡 Hint

Think about atomicity and command order.

Using LPUSH and LTRIM together in a single transaction or pipeline ensures the list is capped immediately after insertion without race conditions.